Do you mean like change the internet Icon from a globe to like a "E"(just and example)??? If thats the case then you would have to create a theme. You would have to search theme builders. You would be able to have what ever for icons! Or just change your theme in general.
Is there a way to do that for 3rd party apps, too? I bought an iBerry skin where you have a bunch of new and shiny icons but the 3rd party icons just screw the whole theme up...
I wish there was a way, tho I'm sure someone out in BB land may be able to pull this one off, Tho I'm not that person. So I would just kinda look around for people that post a lot of themes and ask them! Sorry I couldn't be more help!